Various fields of application require components making it possible to convert optical signals (in general conveyed by optical fibers) into electromagnetic waves that can propagate through empty space. This applies particularly to the field of telecommunications, in which, to connect subscribers to specific services, it is preferable, for reasons related to the system or to the service to be provided, to use radio waves for the last leg from a terminal connected to the optical fiber distribution network. Such a technique will be beneficial for generalizing portable subscriber terminals by fitting flexibly and cheaply into existing radiocommunications infrastructures.
In the field of telecommunications, demand is becoming increasingly high-data rate oriented, whether it be for optical fiber distribution networks or for radio networks for mobile terminals. Essentially for technical reasons to do with passband, it would seem appropriate for those two types of network to converge, with an optical fiber high data-rate fixed network being extended through empty space by a radio broadband access network operating at a very high frequency, i.e. typically a few tens of GHz (e.g. see references 1, 2, 3, 4, 5, 6, 7!).
In such a radio broadband access network at the ends of optical fibers, the radio coverage is provided by base stations distributed outside or inside buildings. Each of the base stations is connected via optical fibers to an exchange. In particular, it has been demonstrated that, for the purposes of serving buildings, such architectures are much more advantageous than networks having combined fiber-and-coaxial cable links or even fiber links to the subscriber Ref. 6, 7!. Such high data-rate radio links grafted onto an optical fiber distribution network not only offer all of the advantages that are related to mobility, but also enable major savings to be made on terminal wiring.
In architectures currently being researched Ref. 3, 4, 5!, the radio signal, generated in an exchange, is conveyed in optical form by the optical fiber distribution network towards the stations of the access network, in which stations optical-to-radio conversion is performed to provide the link to the subscribers. Such an exchange-to-subscriber path is generally termed the "down" path.
The design of the opposite, subscriber-to-exchange, or "up" path is more complex. The information arriving at each station of the access network must be converted into optical form. This is the same function as is performed on the down path, where it is centralized for reasons of economy, but in general up-path conversion takes place at data rates that are lower.
A difficulty encountered by attempts to develop such future networks is the problem of providing reliable and cheap active components for radio terminals.
A certain number of components have already been developed, making it possible to perform the optical wave-to-millimetric wave conversion function separately for the down path from the exchange and for the up path returning from the station. Such a component is constituted by a hybrid optical-millimetric duplexer transposer incorporating unit components of the following types in the same module: detector, oscillator, coupler, light source, and modulator. Assembling such a complex set of components has a considerable impact on the overall cost of the terminal.
In a radio-over-fiber link of the type described in the above-mentioned documents, the exchange is connected by optical fiber to a certain number of stations each of which is equipped with an antenna.
For the down path, the radio signal is applied to the optical carrier at the exchange.
Two approaches are under consideration, depending on whether or not the signal includes the radio carrier. At the station, the radio signal extracted from the optical carrier feeds the antenna which communicates by radio through empty space with the mobile terminals. If it exists on the optical carrier, the radio carrier comes directly from optical-to-radio conversion. Otherwise, it is generated by a local oscillator.
For the up path, the radio signals picked up by the antenna of the station modulate an optical carrier generated by a light source. The resulting optical wave is then taken to the exchange over a fiber that is different from the fiber used for the down path, or possibly even over the same fiber.
At the receiver end of the down path, at the station at the end of the optical fiber, the function to be performed is optical-to-millimetric conversion.
It can be performed merely by photodetecting the optical signal coming from the optical fiber, either in an ultra-fast photodiode followed by transistor amplification 8! or directly in a phototransistor, which makes it possible in addition to provide gain in a more integrated manner 9!. Implementing that solution has shown that the radio signal power extracted from the optical carrier remains rather low in spite of the phototransistor. To provide a power level that is high enough for application to the antenna, a microwave amplifier must be added which is complex and costly, especially at high frequencies, such as those planned in such systems.
To mitigate that limitation, consideration is currently being given to an alternative approach that is more advantageous as regards power. It consists in using an optically controlled millimetric oscillator. It would appear to be cheaper provided that sufficiently powerful oscillators can be implemented cheaply, e.g. with unitary components using technology that is much easier than transistors for millimetric amplifiers. Several works have been published on millimetric sources controlled by optical signals 10, 11, 12, 13!. More particularly, the principle of using the high current gain of a 1.3 -.mu.m light-sensitive phototransistor by integrating it in an oscillator circuit having high output power and possessing a wide locking range at low incident optical power has been demonstrated 14!. Unfortunately, the frequencies obtained in that way remain rather low for the moment.
The above-described approaches handle optical functions separately from electrical functions. An original solution associating both types of function in a common component has been proposed and recently tested successfully in a system experiment 16!. It consists in using a superlattice millimetric diode having negative differential conductance 15, 16!.
At the transmitter for the up path, in the end station at the end of the optical fiber, the function to be performed is an electrical-to-optical conversion. On that path, the electrical signal is assumed to result from demodulation, and it is therefore in base band. That solution, currently being experimented at system level, uses a semiconductor laser 16!. The laser is modulated directly by the electrical signal coming from the antenna, and it returns the information to the exchange at a wavelength identical to or different from that of the down path, depending on the chosen coding. The structures are nevertheless quite complex and are implemented using light-guide technology which poses assembly and cost problems.
The major drawback of the above-mentioned state-of-the-art system is its complexity related to the number of sophisticated components it requires, with obvious consequences on its overall cost. A point to be emphasized more particularly is that the functions are totally separated on the two paths: each function is performed by a component which must be both electrically and optically inserted in the flow of information. The costs of assembly and interconnection (in particular for optical technology) form a large portion of the overall cost of the system, especially since the components usually implemented (lasers of modulators) operate under guided propagation conditions.